After implantation, finger-like projections (chorionic villi) appear on the trophoblast which is surrounded by the uterine tissue and maternal blood.
The chorionic villi & uterine tissue become interdigitated with each other and form placenta. It is a structural and functional unit between the embryo (foetus) and the maternal body.
The placenta is connected to the embryo by an umbilical cord. It transports substances to and from the embryo.
Functions of the placenta:
Acts as barrier between the foetus and mother.
Soluble inorganic and organicmaterials, nutrients, hormones, antibodies, etc can pass through the placenta from the mother to foetus.
Helps in the gas exchange between mother and foetus.
Elimination of nitrogenous wastes of foetus.
Acts as an endocrine gland by secreting Human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, progesterone and relaxin.
During pregnancy, levels of estrogens, progestogens, cortisol, prolactin, thyroxine etc are also increased in maternal blood. They support the fetal growth, metabolic changes in the mother and maintain pregnancy.
The germ layers give rise to all tissues (organs). The stem cells in inner cell mass have the potency to give rise to all the tissues and organs.
